With the widespread of smaller and lighter weight electronic equipment such as cell phones and notebook computers in recent years, demand is growing for higher capacity secondary batteries as the power sources therefor. Non-aqueous electrolyte secondary batteries, which include: a positive electrode whose active material is a lithium cobalt oxide (e.g., LiCoO2); and a negative electrode comprising a carbon material, have been developed for such application and are now widely used.
LiCoO2, however, is very costly since it contains Co. For this reason, as an alternative to LiCoO2, various other metal oxides are proposed and vigorously studied. Examples of such metal oxides include: LiNiO2; LiNi1-xCoxO2 obtained by partially replacing Ni in LiNiO2 with Co; and LiMn2O4.
Particularly, a positive electrode whose active material is a lithium composite oxide containing nickel as an essential element (hereinafter simply referred to as a nickel-containing oxide), such as LiNiO2 and LiNi1-xCoxO2, can offer higher energy density than a positive electrode whose active material is a lithium cobalt oxide. Accordingly, the use of positive electrode comprising a nickel-containing oxide enables low cost production and provides a non-aqueous electrolyte secondary battery with improved capacity characteristic.
However, the positive electrode comprising a nickel-containing oxide has lower thermal stability than the positive electrode comprising a lithium cobalt oxide, and thus the resulting battery has the disadvantage of poor safety.
As one approach for improving safety, Japanese Laid-Open Patent Publication No. 2000-195557 (hereinafter referred to as Patent Document 1) proposes a battery that satisfies a relation: 0.2<C3/C1<0.8 in a region of 45<P/S, where P (mAh) represents battery nominal capacity, S (cm2) represents battery surface area, C1 (mAh) represents discharge capacity discharged at P (mA), and C3 (mAh) represents discharge capacity discharged at 3×P (mA). This publication also proposes to produce a positive electrode whose active material layer has a density of not less than 3.2 g/cm3 in order to achieve the above relation.
As another approach, although silent on positive electrode active material, Japanese Laid-Open Patent Publication No. 2000-123870 (hereinafter referred to as Patent Document 2) proposes a solvent for non-aqueous electrolyte comprising ethylene carbonate and methyl ethyl carbonate at a volume percentage of not less than 10% and not greater than 30%, and not less than 50% and not greater than 90%, respectively. According to this publication, when the content of ethylene carbonate is less than 10%, the effect of forming a protection film on the surface of negative electrode active material is reduced. When the content of methyl ethyl carbonate, which has low viscosity and a low boiling point, exceeds 90%, the resulting battery may generate heat due to a short-circuit, increasing the possibility of explosion.
It is generally accepted that the positive electrode whose active material is a nickel-containing oxide containing nickel as an essential element lacks safety because the active material has poor thermal stability. If a short-circuit occurs inside a fully charged battery, a large current will flow locally into the shorted area, thereby generating heat due to Joule heat.
In nail penetration test (one of the safety tests) in which a nail is penetrated through a battery, the most dangerous situation occurs when a positive electrode current collector and a negative electrode material mixture layer come into contact with each other. When a nail is penetrated through a battery, the positive electrode active material layer separates from the positive electrode current collector at the penetrated area, allowing the bare current collector to come into contact with the negative electrode active material layer through the nail, causing excessive heat generation.
In a battery comprising a nickel-containing oxide, it is difficult to prevent excessive heat generation under such conditions as in nail penetration test only by increasing the density of the positive electrode active material to a certain level as disclosed by Patent Document 1. Moreover, as described in Patent Document 1, increasing the density of the positive electrode active material results in a decrease in high rate discharge performance.
Likewise, only combining a battery whose positive electrode active material is a nickel-containing oxide with the electrolyte disclosed by Patent Document 2 (i.e., the electrolyte comprising ethylene carbonate and methyl ethyl carbonate at a volume percentage of not less than 10% and not greater than 90%, respectively) cannot prevent excessive heat generation. Nickel-containing oxides have the disadvantage of low thermal stability. Although the cause has not been determined, the following inherent characteristics of nickel-containing oxides are presumed responsible for low thermal stability. Specifically, a high valence metal oxide decomposes at high temperatures and releases oxygen. The thermal decomposition temperature of positive electrode active material tends to be influenced by the charge state of battery (i.e., the amount of lithium contained in positive electrode). In other words, positive electrode active material tends to decompose more easily as the amount of lithium is reduced. In a nickel-containing oxide, because the amount of lithium is small, the crystal tends to be unstable. When a nickel-containing oxide and a cobalt-containing oxide both having an equal amount of lithium are compared, nickel-containing oxide is thermodynamically more unstable and thus easily releases oxygen.